'DO me a favour…' : two different meanings?

[Tamara]

Hi

Let’s imagine two people, one of which gives to another some unbelievable reason for not doing something (as promised and/or expected).

And another person pronounces – with a very special voice and emphasis –
"Oh… Do me a favour…’"

Could you give an equivalent phrase for the meaning?
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It’s impossible to learn swimming without entering the water…

[Alan]

Morning Tamara,

The way you are suggesting 'Do me a favour' is spoken in your example suggests sarcasm and really means 'I don't need your help, thank you'. Another expression? Well, how about:

You don't say!
Oh, really!
Well, I never!
Tell me something I don't know!
Really?
Are you telling me?
I should be so stupid?

All the above said in a sarcastic way. To me they all hint at the sort of expressions used in that undefinable and delighttful way that you hear in Jewish humour.

A
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[Tamara]

Morning Alan,

Thanks a lot!
Some your phases are actually very close to what I supposed (i.e. something like the sarcastic and rhetorical :) question: 'Don’t you think I’m so stupid?...' )

Could I also suggest the phrase
Oh, come off it!…

as a possible member of your list?
_________________
It’s impossible to learn swimming without entering the water…